Physical vapor deposition (PVD) is an easy-to-operate process that consumes little raw material and causes little environmental pollution. Moreover, the film formed via PVD is dense and even, and is tightly bonded to the base. With these advantages, PVD is increasingly applied in fabrication of semiconductor elements, in particular, to fabrication of bottom layers of epitaxial wafer. For example, by depositing an AlN layer as the buffer layer, defects from lattice mismatch and thermal mismatch between the substrate and the epitaxial layer can be minimized, and the stress caused therefrom can be eased, thus improving quality of the semiconductor element.
Compared with chemical vapor deposition (CVD), atoms sputtered from PVD have higher energy (in general 10˜20 eV) and migration ability. Therefore, a high-quality buffer layer can be deposited via PVD. In particular, for materials with strong bond energy like AlN, film layers deposited via PVD are better than those deposited with MOCVD. However, direct growth of AlN on the Si substrate surface via PVD is likely to form an amorphous silicon oxide layer at the contact surface between the substrate and AlN, leading to poor quality of subsequently-formed AlN layer and the defects below: as a stress buffer layer, the AlN layer has inconsistent stress release ability, and the stress is concentrated in the amorphous silicon nitride layer of the contact layer, resulting in cracks that degrade quality of semiconductor elements; in addition, the AlN layer with lower density would generate holes, through which, Gallium would corrode the Si substrate in subsequent chemical vapor deposition of Ga-containing nitride. This would form pits and lead to poor performance of semiconductor devices.